95568 complains that CTAD for aggregates doesn't work within
requires-clause and it turned out that it doesn't work when we try
the deduction in a template. The reason is that maybe_aggr_guide
creates a guide that can look like this
template<class T> X(decltype (X<T>::x))-> X<T>
where the parameter is a decltype, which is a non-deduced context. So
the subsequent build_new_function_call fails because unify_one_argument
can't deduce anything from it ([temp.deduct.type]: "If a template
parameter is used only in non-deduced contexts and is not explicitly
specified, template argument deduction fails.")
In such a case we probably want to return PTYPE and try again later like
we do when trying to deduce from a type-dependent expression. Instead
of playing with build_new_function_call with only tf_decltype or with
uses_deducible_template_parms I'm only checking processing_template_decl
because it's cheaper and because when we arein a template it seems
pretty much guaranteed that the guide will have a DECLTYPE_TYPE
parameter.
Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
gcc/cp/ChangeLog:
PR c++/95568
* pt.c (do_class_deduction): When attempting CTAD for aggregates
in a template, return PTYPE.
gcc/testsuite/ChangeLog:
PR c++/95568
* g++.dg/cpp2a/class-deduction-aggr5.C: New test.
---
gcc/cp/pt.c | 9 ++++++++-
.../g++.dg/cpp2a/class-deduction-aggr5.C | 20 +++++++++++++++++++
2 files changed, 28 insertions(+), 1 deletion(-)
create mode 100644 gcc/testsuite/g++.dg/cpp2a/class-deduction-aggr5.C
diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index 53a64c3a15e..1f6ad8289bd 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -28830,7 +28830,14 @@ do_class_deduction (tree ptype, tree tmpl, tree init,
}
if (tree guide = maybe_aggr_guide (tmpl, init, args))
- cands = lookup_add (guide, cands);
+ {
+ /* In a template, GUIDE's type can have DECLTYPE_TYPE parameters
+ and those are non-deduced contexts. Wait until they have been
+ substituted. */
+ if (processing_template_decl)
+ return ptype;
+ cands = lookup_add (guide, cands);
+ }
tree call = error_mark_node;
diff --git a/gcc/testsuite/g++.dg/cpp2a/class-deduction-aggr5.C
b/gcc/testsuite/g++.dg/cpp2a/class-deduction-aggr5.C
new file mode 100644
index 00000000000..01253f42006
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/class-deduction-aggr5.C
@@ -0,0 +1,20 @@
+// PR c++/95568
+// { dg-do compile { target c++20 } }
+
+template<typename T> struct X { T x; };
+template<typename T, typename U> struct X2 { T x; U y; };
+template<typename T> concept Y = requires { X{0}; };
+
+template<typename T>
+void g()
+{
+ X{0};
+ X2{1, 2.2};
+ Y auto y = X{1};
+}
+
+void
+fn ()
+{
+ g<int>();
+}
base-commit: 6b161257f9f8c7a26b7d119ebc32cbbc54d2e508
--
Marek Polacek • Red Hat, Inc. • 300 A St, Boston, MA
